Research On Characteristics Of Offshore Wind Power Sending Through Hybrid HVDC

Conventional HVDC have been widely used since it can transmit a large amount of electrical energy over a long distance. As using semi-controllable devices, this system needs to absorb a lot of reactive power and presents commutation failures easily. With the development of power electric devices, VSC (Voltage Source Converter)-HVDC has shown its characters that the converter can control the active and reactive power independently, and transmit electrical energy to weak systems or loads, but the investment of the converters is high. Hybrid HVDC composed by LCC-HVDC and VSC-HVDC, can combine the advantages of VSC such that controlling the active and reactive power independently, and reduce the investment compared to VSC-HVDC. Hybrid HVDC is applicable for the field of transmitting new energy, this paper presents operation and control strategies of Hybrid HVDC based on direct-drive permanent magnet synchronous generators (PMSG), which adopts Line-commutated Converter (LCC) composed of thyristors in the sending end and VSC in the receiving end, and analysis the fault conditions of this model based on the simulation software PSCAD/EMTDC. The simulation results show that the system can resume the normal operation when the faults disappear. In addition, this paper proposes a new topology for DFIG (double-fed induction generator)-based offshore wind power sending through Hybrid HVDC: VSC in the sending end and LCC in the receiving end, and pointing at the topology, it proposes a coordinated control strategy for WFVSC (wind farm side VSC) and an improved control strategy in order to reduce commutation failures when ground faults occur in the inverter end as the AC system is weak. The topology of Hybrid HVDC connected with wind farm is simulated based on PSCAD/EMTDC. The simulation results verify that WFVSC has power tracking capabilities and the improved control strategy can suppress the number of commutation failures. The two models can combine the advantages of the LCC-HVDC and VSC-HVDC, and have the application prospect in the transmission of offshore power.